Condensate can go to a sink waste only when the connection has a trap, so drain smell cannot blow back through the indoor unit into the room, and when sink water cannot run back into the air conditioner. An open hose pushed into the waste does both things badly. A competent installer or plumber should make the connection, with an air break or tundish where the design needs one. This page does not give a fitting recipe. The route belongs on the quote.
Foul air up the condensate tube
A sink waste is connected to the foul drainage. Without a trap, air from that drainage has a path up the condensate tube into the wall unit and then into the room. That is the smell people blame on the air conditioning after a botched tie-in.
A trap, a tundish, and sink water that stays in the sink
The sink also discharges suddenly. If the condensate hose sits in the flow, dirty water can be pushed back toward the indoor pan. The connection has to prevent that. A professional uses a trapped arrangement and, where needed, a tundish or air break so the two waters meet safely. A hose stuffed in the waste is the failure mode. The connection is a competent person's job. This page does not give a fitting recipe.
The indoor drain still needs a fall to the connection, or a pump whose lift matches that model. The amount of water is whatever a humid cooling day produces, not a rare drip. The waste must accept that trickle without relying on the sink being empty. In heating, the indoor condensate usually reduces and the outdoor unit defrosts instead.
The F-gas company owns the refrigerant side. The waste connection has to meet ordinary plumbing practice and the building's drainage. One firm can do both if they are competent. Ask which part they are including. A price that says 'drain to nearby sink' without mentioning a trap is incomplete. Energy Saving Trust describes the heat pump. It does not describe your trap.
A shared stack in a flat
In a flat, a shared stack may need the freeholder's consent before anyone cuts in. The route still belongs on the price, with the trap named.
Frequently asked questions
Can I push the condensate hose into the sink waste?
No. The connection needs a trap so smell cannot blow back, and it must stop sink water entering the air conditioner. Have a competent person make it.
Why does the room smell when the unit runs?
A drain tied into a waste with no trap can pull foul air back through the indoor unit. That wants a proper trapped connection, not a fragrance spray.
Is a tundish always required?
The design should include one where a break is needed to stop backflow. This page will not give a fitting recipe. Ask the installer to state what they are fitting and why.
Can outdoor defrost water share the sink waste?
Do not connect outdoor defrost water to a foul sink waste as if it were indoor condensate, unless a proper design says so. Defrost water is a different stream, outdoors.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.